The present invention relates to plasma processes used in the fabrication of semiconductor integrated circuits on semiconductor wafer substrates. More particularly, the present invention relates to a method for monitoring plasma parameters during a plasma process and in-situ termination or modification of the process in the event that the plasma parameters fall outside plasma parameter specifications.
Integrated circuits are formed on a semiconductor substrate, which is typically composed of silicon. Such formation of integrated circuits involves sequentially forming or depositing multiple electrically conductive and insulative layers in or on the substrate. Etching processes may then be used to form geometric patterns in the layers or vias for electrical contact between the layers. Etching processes include xe2x80x9cwetxe2x80x9d etching, in which one or more chemical reagents are brought into direct contact with the substrate, and xe2x80x9cdryxe2x80x9d etching, such as plasma etching.
Various types of plasma etching processes are known in the art, including plasma etching, reactive ion (RI) etching and reactive ion beam etching. In each of these plasma processes, a gas is first introduced into a reaction chamber and then plasma is generated from the gas. This is accomplished by dissociation of the gas into ions, free radicals and electrons by using an RF (radio frequency) generator, which includes one or more electrodes. The electrodes are accelerated in an electric field generated by the electrodes, and the energized electrons strike gas molecules to form additional ions, free radicals and electrons, which strike additional gas molecules, and the plasma eventually becomes self-sustaining. The ions, free radicals and electrons in the plasma react chemically with the layer material on the semiconductor wafer to form residual products which leave the wafer surface and thus, etch the material from the wafer.
In the fabrication of semiconductor devices, particularly sub-micron scale semiconductor devices, profiles obtained in the etching process are very important. Careful control of a surface etch process is therefore necessary to ensure directional etching. In conducting an etching process, when an etch rate is considerably higher in one direction than in the other directions, the process is called anisotropic. A reactive ion etching (RIE) process assisted by plasma is frequently used in an anisotropic etching of various material layers on top of semiconductor substrate. In plasma enhanced etching processes, the etch rate of a semiconductor material is frequently larger than the sum of the individual etch rates for ion sputtering and individual etching due to a synergy in which chemical etching is enhanced by ion bombardment.
To avoid subjecting a semiconductor wafer to high-energy ion bombardment, the wafer may also be placed downstream from the plasma and outside the discharge area. Downstream plasma etches more in an isotropic manner since there are no ions to induce directional etching. The downstream reactors are frequently used for removing resist or other layers of material where patterning is not critical. In a downstream reactor, radio frequency may be used to generate long-lived radioactive species for transporting to a wafer surface located remote from the plasma. Temperature control problems and radiation damage are therefore significantly reduced in a downstream reactor. Furthermore, the wafer holder can be heated to a precise temperature to increase the chemical reaction rate, independent of the plasma.
In a downstream reactor, an electrostatic wafer holding device known as an electrostatic chuck is frequently used. The electrostatic chuck attracts and holds a wafer positioned on top electrostatically. The electrostatic chuck method for holding a wafer is highly desirable in the vacuum handling and processing of wafers. An electrostatic chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure, in contrast to a conventional method of holding wafers by a mechanical clamping method.
Referring to the schematic of FIG. 1, a conventional plasma etching system is generally indicated by reference numeral 10. The etching system 10 includes a reaction chamber 12 having a typically grounded chamber wall 14. An electrode, such as a planar coil electrode 16, is positioned adjacent to a dielectric plate 18 which separates the electrode 16 from the interior of the reaction chamber 12. A second electrode 20 is provided in the bottom portion of the reaction chamber 12. Plasma-generating source gases are introduced into the reaction chamber 12 by a gas supply (not shown). Volatile reaction products and unreacted plasma species are removed from the reaction chamber 12 by a gas removal mechanism, such as a vacuum pump (not shown).
The dielectric plate 18 illustrated in FIG. 1 may serve multiple purposes and have multiple structural features, as is well known in the art. For example, the dielectric plate 18 may include features for introducing the source gases into the reaction chamber 12, as well as those structures associated with physically separating the electrode 16 from the interior of the chamber 12.
Electrode power such as a high voltage signal, provided by a power generator such as an RF (radio frequency) generator (not shown), is applied to the electrode 16 to ignite and sustain a plasma in the reaction chamber 12. Ignition of a plasma in the reaction chamber 12 is accomplished primarily by electrostatic coupling of the electrode 16 with the source gases, due to the large-magnitude voltage applied to the electrode 16 and the resulting electric fields produced in the reaction chamber 12. Once ignited, the plasma is sustained by electromagnetic induction effects associated with time-varying magnetic fields produced by the alternating currents applied to the electrode 16 and the electrode 20. The plasma may become self-sustaining in the reaction chamber 12 due to the generation of energized electrons from the source gases and striking of the electrons with gas molecules to generate additional ions, free radicals and electrons. A semiconductor wafer 22 is positioned in the reaction chamber 12 and is supported by the electrode 20. The electrode 20 is typically electrically-biased by a bias voltage 24 to provide ion energies that are independent of the RF voltage applied to the electrode 16 and that impact the wafer 22.
In the etching of conductive and insulative layers on wafer substrates, chamber condition monitoring is important to prevent drifting of plasma parameters, particularly plasma electron density and electron collision rate, outside of preset parameter specifications. Traditional monitoring techniques include the use of monitor wafers to probe the etch rate and uniformity variation for a particular plasma process. However, this method is time-consuming and costly. Moreover, the data obtained through use of the monitor wafers may not be sufficiently sensitive to detect crucial deviations in process parameters from the specification. Accordingly, a cheaper and more accurate method of monitoring plasma parameters in a plasma process is needed.
As shown in FIG. 2, a strong correlation exists between the etch rate of a polysilicon layer (indicated by the connected circles) and the electron collision rate (indicated by the connected triangles). The plasma electron collision rate is proportional to the plasma electron density. Accordingly, it is proposed that maintaining the plasma electron density and electron collision rate within upper and lower limits during a plasma process can facilitate optimal plasma etching or plasma-mediated material deposition.
An object of the present invention is to provide a novel method for monitoring plasma parameters in a plasma process.
Another object of the present invention is to provide a novel and cost-effective method for monitoring plasma parameters in a plasma process.
Still another object of the present invention is to provide a novel and accurate method for monitoring plasma parameters in a plasma process.
Yet another object of the present invention is to provide a method for in-situ monitoring and control of plasma parameters during a plasma process.
Another object of the present invention is to provide a method for in-situ monitoring of the electron density and/or collision rate of electrons in a plasma during a plasma process in order to either maintain either or both of these plasma parameters within predetermined parameter specifications or terminate the plasma process in the event that the monitored plasma parameter or parameters drift outside the specifications.
A still further object of the present invention is to provide a method which monitors both plasma parameter data and tool parameter data from a plasma process tool, compares the parameters to preset parameter specifications, continues the plasma process in the event that the parameters remain within the specifications, and terminates the plasma process in the event that the parameters drift outside the specifications.
Yet another object of the present invention is to provide a method which measures plasma parameter data from a plasma process tool during a plasma process, compares the obtained data to preset plasma parameter specifications, and adjusts or modifies the process recipe or season procedure.
In accordance with these and other objects and advantages, the present invention is generally directed to a method for monitoring plasma parameters during a plasma process such as a plasma etching process or plasma deposition process, comparing the measured plasma parameters to predetermined parameter specifications, and either terminating the plasma process or modifying the plasma process in progress to re-establish the plasma parameters within the parameter specifications. The plasma parameters may be measured by the self-excited electron resonance spectroscopy (SEEKS) technique or by microwave interferometry. In one embodiment, both plasma parameter data and tool parameter data are obtained from a plasma process tool, and these parameters are compared to preset parameter specifications. The plasma process is continued in the event that the obtained parameters remain within the parameter specifications, and is terminated in the event that the parameters drift outside the specifications. In another embodiment, the plasma parameter data obtained from the plasma process tool during the plasma process is constantly compared to preset plasma parameter specifications, and the process recipe or season procedure for the plasma process is adjusted to bring the plasma parameters back to within the specifications.